High intensity discharge (HID) lamps are commonly used in large area lighting applications, due to their high energy efficiency and superb long life. The existing HID product range consists of mercury vapor (MV), high pressure sodium (HPS), and quartz metal halide (MH) lamps. In recent years, ceramic metal halide lamps (for example, Philips MasterColor® series) have entered the market place. Compared to the conventional HID lamps, the ceramic metal halide lamps display excellent initial color consistency, superb stability over life (lumen maintenance >80%, color temperature shift <200 K at 10,000 hrs), high luminous efficacy of >90 lumens/watt and a lifetime of about 20,000 hours. These highly desirable characteristics are due to the high stability of the polycrystalline alumina (PCA) envelopes and a special mixture of salts, which emits a continuous-spectrum light radiation close to natural light.
The salt mixture used in Philips MasterColor® series lamps is composed of NaI, CaI2, TlI, and rare-earth halides of DyI3, HoI3 and TmI3 NaI, CaI2 and TlI are mainly for emitting high intensity line radiation at various colors, but they also contribute to continuous radiation. The rare-earth halides are for continuous radiation throughout the visible range, resulting in a high color rendering index (CRI). By adjusting the composition of the salts, color temperatures of 3800-4500 K, and a CRI of above 85 can be achieved. The existing power range of such lamps is from 20 W to 150 W. The relatively narrow power range makes these products only suitable for the applications requiring low power installations, such as most indoor low-ceiling retail spaces. For large area, higher power applications requiring a lamp power of 200 W to 1000 W, the primary available products are MV, HPS and MH lamps.
One example of a lamp of the kind set forth is known from U.S. Pat. No. 5,424,609. The known lamp has a comparatively low power of 150 W at the most at an arc voltage of approximately 90 V. Because the electrode in such a lamp conducts comparatively small currents during operation of the lamp, the dimensions of the electrode may remain comparatively small so that a comparatively small internal diameter of the projecting plug suffices. In the case of a lamp having a rated power in excess of 150 W, or a substantially lower arc voltage, for example as in the case of large electrode currents, electrodes of larger dimensions are required. Consequently, the internal plug diameter will be larger accordingly. It has been found that in such lamps there is an increased risk of premature failure, for example due to breaking off of the electrode or cracking of the plug.
There is a need in the art for HID lamps of the ceramic metal halide type with power ranges of about 150 W to about 1000 W.